The present invention is related to a vertical-cavity, surface-emission-type laser diode and the process of making the same. Further, the present invention relates to a vertical-cavity, surface-emission-type laser-diode array, an optical transmission module, an optical transceiver module and also an optical telecommunication system.
Vertical-cavity, surface-emission-type laser diode is a laser diode that emits an optical beam in a vertical direction to a substrate. It is used for a light source of optical interconnection systems and optical pickup devices, and the like.
A vertical-cavity, surface-emission-type laser diode has an active region including an active layer that produces a laser beam. The active region is sandwiched with a pair of reflectors, wherein a semiconductor distributed Bragg reflector, in which a low-refractive index layer and a high refractive index layer are laminated alternately, is used widely for the reflectors. Materials having a wider bandgap than the active layer and not causing absorption of the optical beam coming from an active layer are used for the semiconductor distributed Bragg reflector. Particularly, the materials that achieve a lattice matching with the substrate are used so as to avoid lattice relaxation.
Meanwhile, the reflector has to have a high reflectance of 99% or more. Generally, the reflectance of the reflector becomes higher by increasing the number of stacking. However, production of the vertical-cavity, surface-emission-type laser diode becomes difficult when the number of stacking in the reflectors is increased excessively. Because of this, it is preferable that there exists a large refractive index difference between the low-refractive index layer and the high refractive index layer constituting the semiconductor distributed Bragg reflectors. AlAs and GaAs are end-member compositions of the system AlGaAs having a lattice constant almost the same as that of GaAs. Further, the materials of this system can provide a large refractive index difference therebetween. Because of this reason, it is possible to achieve a high reflectance with fewer number of stacking by using the material of the AlGaAs system. Thus, the material of the AlGaAs system is used widely.
However, the material containing Al is very reactive, and crystal defects, originating from Al, are formed easily. For example, oxygen molecules or water molecules contained in the source material or growth atmosphere are easily incorporated into the crystal as a result of reaction with Al. Once they are thus incorporated, they form a crystal defect acting as non-optical recombination center, resulting in degradation of efficacy of optical emission. Further, there is a concern that the reliability of the device may be degraded due to the existence of such crystal defects.
Even when the active region is formed by a layer not containing Al, the problem of non-optical recombination still occurs when Al is contained in the low-refractive index layer (formed of a widegap layer) of the reflector that makes a contact with the active region. More specifically, such a non-optical recombination may occur at the interface between the active region and the reflector when carriers are injected for recombination. Thereby, the efficacy of optical emission falls off inevitably. In order to avoid this adversary influence, it is necessary to carry out rigorous process control, material purity control, optimization of growth condition, and the like. Still, it is not easy to produce a device with high quality.
Meanwhile, there are proposals in Japanese Laid-Open Patent Applications 08-340146 and 07-307525 to form a semiconductor distributed Bragg reflector by using GaInP and GaAs, which are free from Al. However, the difference of refractive index between GaInP and GaAs is only one-half the refractive index difference between AlAs and GaAs. Because of this, the number of stacking in the reflector has to be increased significantly, and the production of the laser diode becomes difficult. Associated with this there arise various problems such as degradation of yield, increased device resistance, increased time needed for producing a laser diode. Further, because of the increase of total thickness, there appears a difficulty in providing electric interconnection in such a laser diode.
Meanwhile, it is practiced to use a current confinement structure in the art of laser diode for reducing the threshold of laser oscillation. Japanese Laid-Open Patent Application 7-240506 discloses a structure that uses a current confinement structure including a high resistance layer formed by an ion implantation process in combination with a semiconductor distributed Bragg reflector that consists of AlAs/GaAs. Further, Japanese Patent 2,917,971 proposes a vertical-cavity, surface-emission-type laser diode that uses, in addition to an optical cavity formed by the semiconductor distributed Bragg reflectors of the AlGaAs/GaAs stacked structure, a current confinement structure that includes an oxide film formed by selective oxidization of a part of the Al(Ga)As optical cavity structure. In this proposal, the oxidation is conducted by supplying water vapor of high temperature. It should be noted that the oxidation process using water vapor of high temperature is capable of forming a true insulator of AlxOy. According to such an approach, the distance between the active layer and the current confinement layer can controlled exactly by controlling the process of crystal growth. Further, it is possible narrow the current path significantly. In view of these, the foregoing construction is suited for reducing reactive current and for reducing the active region. Because of this, it is also suited to for reducing electric power consumption. Thus, the construction is used widely recently.
It should be noted that the foregoing Japanese Patent 2,917,971 uses the phenomenon that the oxidation rate starts to increase sharply as the Al content in the AlGaAs layer is increased. Thus, in order to ensure that only the part to be oxidized is oxidized, the foregoing process increases the Al content of the layer in which the oxidation is to be caused. In this way, it is possible to obtain a current confinement structure by a selective oxidation process. In view of this, the Al content of the AlGaAs layer forming the low-refractive index layer of the semiconductor distributed Bragg reflector is set smaller (Ga content is increased) than the Al content of the Al(Ga)As/GaAs oxidation layer. The composition of AlxGa1-xAs (x=0.97) is used for the selectively oxidized layer in the foregoing Japanese Patent 2,917,971, while a composition of AlxGa1-xAs (x=0.92) is used for the low-refractive index layer of the semiconductor distributed Bragg reflector.
In the art of forming a current confinement structure by such a selective oxidation process, an approach is adopted to oxidize an AlAs layer from a sidewall surface thereof. In order that such a process is to be conducted, it is necessary to remove unnecessary layers by means of a mesa etching process such that the sidewall surface of the AlAs layer to be oxidized is exposed. However, in view of variation in the etching rate, there may be caused variation of mesa height within a lot. Further, there may be caused a lot-to-lot variation of mesa height. When such a variation has been caused, the device characteristic may be scattered correspondingly.
Current optical-fiber telecommunication technology uses a laser diode of long wavelength band of 1.3 xcexcm or 1.55 xcexcm for utilizing the wavelength slot of quartz optical fibers in which the optical loss is minimum. The optical fiber telecommunication system is spreading rapidly and it is expected that it may reach a subscriber terminal (Fiber To The Home; FTTH) in a near future. Furthermore, the technology of information transmission by way of optical signals is going to be introduced even to a device-to-device interconnection system inside an apparatus or even to an interconnection system inside a device. Like this, the technology of information transmission will become important still more. In order to realize such an optical interconnection system, it is essential to realize an optical telecommunication module of unprecedented low-cost. Thus, there is a keen demand for a small, long wavelength-band laser diode of low electric power consumption, with excellent temperature characteristics, capable of eliminating the need of a cooling system.
Currently, the material of GaInPAs system formed on an InP substrate, which is a group III-V semiconductor material, monopolizes the market. It should be noted that the material of the GaInPAs system can be tuned to the foregoing wavelength band. However, the material of the InP system has a drawback, because of the small discontinuity in the conduction band between the cladding layer (spacer layer) and the active layer, in that the electrons injected into the active layer are poorly confined, particularly when temperature of the device is increased. This results in a decrease of efficiency. Further, the materials that achieve lattice matching with an InP substrate cannot provide large refractive index difference suitable for realizing a semiconductor distributed Bragg reflector. As a result, the vertical-cavity, surface-emission-type laser diode of the long wavelength having a performance suitable for practical use has not been obtained.
The material of the GaInNAs system formed on a GaAs substrate is proposed in the Japanese Laid-Open Patent Application No. 6-37355, as the material that can settle the foregoing problems. It should be noted that GaInNAs is a group III-V mixed crystal containing N in addition to other group V element. In the system of GaInNAs, it is possible to achieve lattice matching with a GaAs substrate by adding N to GaInAs having a lattice constant larger than that of GaAs. By doing so, the bandgap energy is reduced also. Thus, it becomes possible to realize optical emission at the wavelength band of 1.3 xcexcm or 1.5 xcexcm. Kondou, et al., calculated the band lineup of this system in the article, Jpn. J. Appl. Phys. Vol. 35 (1996), pp. 1273-1275. As this is a material that can achieve lattice matching with GaAs, a large band discontinuity can be realized by using AlGaAs for the cladding layer. Because of this, there is an expectation that a laser diode having a high characteristic temperature may be realized by using such a material. Further, it should be noted that the material of GaInNAs can be formed on a GaAs substrate. Thus, it becomes possible to construct a/the semiconductor multilayer reflector by using an Al(Ga)As/GaAs material system. Thereby, it becomes possible to reduce the number of stacking in the multilayer reflector significantly as compared with the case of forming the multilayer reflector on the InP substrate. Further, it becomes possible to form an AlAs selective-oxidation layer as the current confinement structure, and the operating current is reduced effectively.
However, the problem noted above arises in the case the material system of Al(Ga)As/GaAs is used for the semiconductor multilayer reflector, as proposed in the Japanese Laid-Open Patent Application 10-303515 or Japanese Laid-Open Patent Application 11-145560. Further, the problem similar to above arises also in the case an AlAs selective-oxidation layer is used for the current confinement structure.
Accordingly, it is a general object of the present invention to provide a novel and useful vertical-cavity, surface-emission-type laser diode and the process of making the same wherein the foregoing problems are eliminated.
Another and specific object of the present invention is to provide a vertical-cavity, surface-emission-type laser diode having excellent reliability and easily fabricated, without increasing the total thickness thereof.
Another object of the present invention is to provide a vertical-cavity, surface-emission-type laser-diode array, an optical transmission module, an optical transceiver module, and an optical telecommunication system.
Another object of the present invention is to provide a vertical-cavity, surface-emission-type laser diode having an optical cavity structure on or above a semiconductor substrate, the optical-cavity structure comprising an active region containing at least one active layer that produces a laser beam, and upper and lower reflectors sandwiching the active region to form the optical cavity, the lower reflector including a semiconductor distributed Bragg reflector having a refractive index that changes periodically, the lower reflector reflecting an optical beam incident thereto by diffraction, the semiconductor distributed Bragg reflector comprising a low-refractive-index layer of AlxGa1-xAs (O less than xxe2x89xa61) and a high-refractive-index layer of AlyGa1-yAs (0xe2x89xa6y less than xxe2x89xa61), wherein a non-optical recombination elimination layer is provided between the active layer and the lower reflector.
According to the present invention, a non-optical recombination elimination layer is provided between the active layer and the lower reflector in the construction in which the active region (an active layer is included), in which injection of carriers is made, is sandwiched by the upper and lower reflectors. Thus, the phenomenon that the crystal defects that originate from Al crawl up to the active layer at the time of crystal growth is effectively restrained, even in the case the lower reflector is formed of a semiconductor distributed Bragg reflector including a semiconductor layer that contains Al. Thereby, the adversary effect caused by the defects is suppressed, and the active layer can be formed with high crystal quality. Accordingly, non-optical recombination caused by the crystal defects that originate from Al is reduced, and the efficiency of optical emission and the reliability of the laser diode are improved. As compared with the case in which the low-refractive index layers of the semiconductor distributed Bragg reflector are all formed of GaxIn1-xPyAs1-y (0 less than xxe2x89xa61, 0 less than yxe2x89xa61), the semiconductor distributed Bragg reflector of the present invention can maintain a large refractive index difference. It should be noted that the reflector of the present invention is formed mostly of the material of the AlGaAs system. Thus, a reflectance is achieved for the reflectors with fewer number of stacking. Because of this, it is possible to obtain the high reflectance without increasing the number of stacking of in the reflector or increasing the total thickness of the device. In the laser diode of the present invention, the total thickness of the vertical-cavity, surface-emission-type laser diode does not increase, and the operating current is small. Further, the laser diode has excellent reliability. As such, the vertical-cavity, surface-emission-type laser diode can be produced easily.
Another object of the present invention is to provide a vertical-cavity, surface-emission-type laser diode having an optical cavity on or above a semiconductor substrate, the optical cavity comprising an active region containing at least one active layer that produces a laser beam, and upper and lower reflectors sandwiching the active region to form the optical cavity, each of the upper and lower reflectors including a semiconductor distributed Bragg reflector in which a refractive index is changed periodically, the upper and lower reflectors reflecting an optical beam incident thereto, the semiconductor distributed Bragg reflector comprising a low-refractive-index layer of AlxGa1-xAs (0 less than xxe2x89xa61) and a high-refractive-index layer of AlyGa1-yAs (0xe2x89xa6y less than xxe2x89xa61), wherein a non-optical recombination elimination layer is provided between the active layer and the lower reflector and a non-optical recombination elimination layer is provided between the active layer and the upper reflector.
According to the present invention, a non-optical recombination elimination layer is provided between the active layer and each of the lower and upper reflectors in the construction in which the active region (an active layer is included), in which injection of carriers is made, is sandwiched by the upper and lower reflectors. Thus, the phenomenon that the crystal defects that originate from Al crawl up to the active layer at the time of crystal growth is effectively restrained, even in the case the lower reflector is formed of a semiconductor distributed Bragg reflector including a semiconductor layer that contains Al. Particularly, the active region, in which carrier injection occurs, is sandwiched by the non-optical recombination at both top part and bottom part thereof. Thereby, non-optical recombination caused by the crystal defects that originate from Al is reduced particularly effectively, and the efficiency of optical emission and the reliability of the laser diode are improved easily. While the effect of the non-optical recombination elimination layer is obtained when it is inserted to one of the reflectors, the construction in which the non-optical recombination elimination layer is provided to each of the reflectors is extremely effective for eliminating the influence of the Al defects. As compared with the case in which the low-refractive index layers of the semiconductor distributed Bragg reflector are all formed of GaxIn1-xPyAs1-y (0 less than xxe2x89xa61, 0 less than yxe2x89xa61), the semiconductor distributed Bragg reflector of the present invention can maintain a large refractive index difference. It should be noted that the reflector of the present invention is formed mostly of the material of the AlGaAs system. Thus, a reflectance is achieved for the reflectors with fewer number of stacking. Because of this, it is possible to obtain the high reflectance without increasing the number of stacking of in the reflector or increasing the total thickness of the device. In the laser diode of the present invention, the total thickness of the vertical-cavity, surface-emission-type laser diode does not increase, and the operating current is small. Further, the laser diode has excellent reliability. As such, the vertical-cavity, surface-emission-type laser diode can be produced easily.
By using GaxIn1-xPyAs1-y (0 less than xxe2x89xa61, 0 less than yxe2x89xa61) for the non-optical recombination elimination layer in combination with a GaAs substrate, the carriers that cause a leak to the layer containing Al through the GaxIn1-xPyAs1-y (0 less than xxe2x89xa61, 0 less than yxe2x89xa61) is eliminated substantially particularly in the case that the bandgap of the material used for the active layer is smaller than that of GaAs, in view of the fact that the bandgap of the GaxIn1-xPyAs1-y (0 less than xxe2x89xa61, 0 less than yxe2x89xa61) layer, which is substantially free from Al (Al content is 1% or less with regard to the group III elements), is larger than the bandgap of GaAs. Because of this, non-optical recombination can be prevented. Accordingly, a vertical-cavity, surface-emission-type laser diode, operating with small current and having excellent reliability is realized.
In the case the lattice constant of the GaxIn1-xPyAs1-y (0 less than xxe2x89xa61,0 less than yxe2x89xa61) layer is smaller than the lattice constant of the GaAs substrate, the GaxIn1-xPyAs1-y (0 less than xxe2x89xa61,0 less than yxe2x89xa61) layer accumulates a tensile strain therein. Thus, crawling-up of defects from the substrate to a growth layer during a growth process is effectively suppressed. As a result, the efficacy of optical emission is improved. Further, it becomes possible to grow a layer accumulating a compressive strain of 2% or more, for example, as the active layer. Furthermore, it becomes possible to grow a strained layer with a thickness exceeding the critical film thickness.
In view of the fact that the GaxIn1-xPyAs1-y (0 less than xxe2x89xa61, 0 less than yxe2x89xa61) layer contacts with the active region, and in view of the fact that the bandgap energy of the GaxIn1-xPyAs1-y (0 less than xxe2x89xa61, 0 less than yxe2x89xa61) layer become larger with decreasing lattice constant, the hetero-barrier height between the active region and the GaxIn1-xPyAs1-y (0 less than xxe2x89xa61, 0 less than yxe2x89xa61) is increased. Thus, the efficiency of carrier confinement is improved. Thereby, improvement with regard to temperature characteristics and threshold current are achieved.
In the case the lattice constant of the GaxIn1-xPyAs1-y (0 less than xxe2x89xa61, 0 less than yxe2x89xa61) layer is larger than the lattice constant of the GaAs semiconductor substrate, the GaxIn1-xPyAs1-y (0 less than xxe2x89xa61, 0 less than yxe2x89xa61) accumulates a the compressive strain. Thus, crawling up of defects formed during the growth process or existing in the substrate to the growth layer is suppressed, and the efficiency of optical emission is improved. Further, it becomes possible to grow a layer accumulating a compressive strain of 2% or more. Further, it becomes possible to grow a strained layer with a thickness exceeding the critical thickness.
In the case the sense of strain of the GaxIn1-xPyAs1-y (0 less than xxe2x89xa61, 0 less than yxe2x89xa61) layer is the same as the sense of the strain of the active layer, there is an effect, in addition to the above-noted effect of insertion of the strained layer, in that the compressive strain that the active layer senses is reduced substantially. Thus, the adversary effect of the defects existing on the surface of a foundation layer, on which the growth is made, in the state immediately before the start of the growth process is reduced substantially. As a result, the crystal quality of the active layer improved and the characteristics of the laser diode are improved. Especially, this improvement is effective in vertical-cavity, surface-emission-type laser diode of long wavelength band in which growth of thick film is necessary.
The non-optical recombination elimination layer of the GaInPAs system containing P functions as an etching stopper with respect to the layer of the AlGaAs system that contains Al as a principal component. Because of this, the height of the mesa structure provided by a wet etching process for selective oxidation process is controlled exactly. By using the mesa structure, it becomes possible to form a current confinement layer by selectively oxidizing the layer that contains Al and As at the location above the non-optical recombination elimination layer. In this way, the accuracy of process control is improved. Further, the homogeneity and reproducibility is improved with regard to the device characteristics. Furthermore, the yield is improved, and the fabrication cost it reduced.
Further, according to the present invention, it becomes possible to form a vertical-cavity, surface-emission-type laser diode for use in long wavelength band of 0.9xcexcm or more on a GaAs substrate by using any of GaInNAs or GaInAs.
By providing a compressive strain of 2.0% or more to the active layer in the present invention, it becomes possible to realize a vertical-cavity, surface-emission-type laser diode operable at the wavelength hitherto not possible. For example, by using GaInAs for the active layer, it becomes possible to provide a vertical-cavity, surface-emission-type laser diode operable at the wavelength of 1.1 xcexcm or longer. By using GaInNAs for the active layer, the crystal quality of the active layer is improved, and the threshold current density is reduced. Thereby, it becomes possible to provide a vertical-cavity, surface-emission-type laser diode having excellent reliability and still operable at the wavelength band of 1.3 xcexcm or longer.
By arranging such a vertical-cavity, surface-emission-type laser diode in the form of one-dimensional or two-dimensional array, it is possible to provide a vertical-cavity, surface-emission-type laser-diode array with excellent homogeneity and reproducibility. In the case of forming an array, in-plane homogeneity influences the element-to-element variation of characteristics. As noted before, it is possible to use the crystal layer GaInPAs system as an etching stopper with respect to the crystal layer of AlGaAs system. Because of this, the height of the mesa structure used for the selective oxidation process is controlled exactly over the elements included in the array. Because of this, not only the precision of process control at the time of device fabrication is improved, but also the homogeneity of characteristics between the elements in the array and reproducibility of the vertical-cavity, surface-emission-type laser-diode array are improved also.
By using the vertical-cavity, surface-emission-type laser diode or the laser-diode array of the present invention as an optical source, in other words by using the vertical-cavity, surface-emission-type laser diode low-cost, high-quality and excellent reliability for the optical source, a low cost, highly efficient and reliable optical transmission module is realized.
By using the vertical-cavity, surface-emission-type laser diode or laser-diode array of the present invention as an optical source, in other words by using the vertical-cavity, surface-emission-type laser diode low-cost, high-quality and excellent reliability for the optical source, a low cost, highly efficient and reliable optical transceiver module is realized.
By using the vertical-cavity, surface-emission-type laser diode or laser-diode array of the present invention as an optical source, in other words by using the vertical-cavity, surface-emission-type laser diode low-cost, high-quality and excellent reliability for the optical source, a low cost, highly efficient and reliable optical telecommunication, including an optical-fiber telecommunication system and an optical interconnection system, is realized.
By providing a process for removing residual Al source material, residual Al product, residual Al compound or residual Al from a location such as the gas supply line or growth chamber, in which contact with a nitrogen compound source material or impurity included therein tends to occur, in the interval after the growth of the semiconductor layer containing Al but before the start of growth of the active layer that contains nitrogen in the fabrication process of the vertical-cavity, surface-emission-type laser diode, it becomes possible in the present invention to decrease the amount of oxygen taken into the active layer that contains nitrogen during the growth process of the active layer. Thereby, it becomes possible to grow the semiconductor light-emitting device without decreasing the efficiency of optical emission even in the case the active layer containing nitrogen is formed on the upper part of the semiconductor layer containing Al in the semiconductor light-emitting device.
By providing a process for removing residual Al source material, residual Al product, residual Al compound or residual Al from a location such as the gas supply line or growth chamber, in which contact with a nitrogen compound source material or impurity included therein tends to occur, in the interval after the growth of the semiconductor layer containing Al and before the end of growth of the non-optical recombination elimination layer in the fabrication process of the vertical-cavity, surface-emission-type laser diode, it becomes possible in the present invention to decrease the amount of oxygen taken into the active layer that contains nitrogen during the growth process of the active layer. Further, the adversary effect of non-optical recombination originating caused by oxygen taken into the growth interrupt interface at the time electric current is injected for device operation is successfully eliminated. Thereby, it becomes possible to obtain the semiconductor light-emitting device having a high efficiency of optical emission even in the case the active layer containing nitrogen is formed on the upper part of the semiconductor layer containing Al in the semiconductor light-emitting device.
By providing a process for removing residual Al source material, residual Al product, residual Al compound or residual Al from a location such as the gas supply line or growth chamber, in which contact with a nitrogen compound source material or impurity included therein tends to occur, in the fabrication process of the vertical-cavity, surface-emission-type laser diode by an MOCVD process that uses source materials of at least a metal organic Al source and a nitrogen compound source, it becomes possible to improve the efficiency of optical emission of the semiconductor light-emitting device as compared with the case in which no such a removal is made.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.